Electropolishing nickel



April 1, 1958 E. B. SAUBESTRE ET AL 3 2,

7 ELECTROPOLISHING NICKEL Filed'se t. 15, 1954 2 Sheets-Sheet 1 FIG. I.

%SULFURIC 60 VoWATER 70 HYDROCHLORIC AC lD INVENTOR. EDWARD A. GAUBESTEE e w/lv E- Ban 15g MAN ATTORNEY April 1, 1958 E. B. SAUBESTRE ET L v2,829,098

ELECTROPOLISHING NICKEL Filed Sept. 15, 3.954 2 Sheets-Sheet 2 FIG. 2.

"/o SULFURIC AC ID 40 I. 60 WATER HYDROCHLORIC ACID INVENTOR EDWARD b. \SAUBESTRE EDWIN R- BOWERMAN Nda W ATTORNEY United States Patent I 2,829,098 ELECTROPOLISHING NICKEL Edward B. Saubestre, Elmhurst, and Edwin R. Bowerman, Whitestone, N. Y., assiguors to Sylvania Elec tric Products lnc., a corporation of Massachusetts Application September 13, 1954, Serial No. 455,504

5 Claims. (Cl. 204-1405) The present invention relates to electrolytic treatment of metals, and in particular to improved methods and baths for electropolishing nickel. Specifically the invention relates to an aqueous electrolyte consisting essentially of hydrochloric acid, sulfuric acid and water, and to the use of such electrolyte in the anodic polishing of nickel.

Many solutions have been suggestedfor electropolishing nickel parts including sulfuric acid solutions, sulfuricphosphoric acid mixtures, and chloride-containing solutions. The mentioned solutions as well as others suggested for the electrolytic working of nickel have found only limited application. 'Study of the anodic behavior of nickel in various chlorides has indicated that under proper but rather critical conditions nickel can be made to polarize in chloride-containing solutions with resulting polishing action. It has been found that for predetermined concentrations polishing action can be obtained in hydrochloric, sodium chloride, aluminum chloride and magnesium chloride solutions at current densities of the order of 600 to 2500 amperes per square foot. The aluminum chloride and magnesium chloride solutions exhibit chemical drawbacks particularly from the'standpoint of conductivity and solubility, while theuse of concentrated hydrochloric acid solutions causes severe pitting along with the brightening action. Other chloridecontaining electrolytes, such as urea-ammonium chloride baths have been suggested in the literature but likewise find limited application as they are fused salt melts- Although nickel may be electropolished in a bath containing urea-ammonium chloride and anhydrousnickel chloride, there are definite limits in permissible current densities and in the range of current densities. current densities in excess of the order of 40 amps per square foot leads to gas evolution and increased tendency to pitting. Still higher current densities produce filming on the metal surfaces to disadvantage especially when incorporating the electropolished components in electrical or electronic devices.

Accordingly, it isan object of the present invention to provide an improved method of electrochemically working nickel obviating one or more of the aforesaid difliculties. Specifically it is within the contemplation of the present invention to provide improved methods and baths for anodically polishing nickel components.

For widespread adoption of an electrolyte for polishing nickel, the electrolyte should exhibit one or more of the following characteristics. A preferred electrolytic solution should be capable of operating at room temperatures, be relatively non-critical with respect to temperature control, be highly efficient, have a rapid treatment rate, exhibit a relatively broad operating range of anode current densities such that irregular objects may 7 be readily polished, and leave little or no interfering films on the surfaces'after treatment. v Therefore, it is a still further object of the present invention to provide improved'electrolytic baths and meth- The use of ods for polishing nickel parts which are not extremely ar I 1C &

critical with respect to the operating temperature of the baths, which facilitate electropolishing at room temperatures, and which require little or no control over ambient temperature.

it IS a provide nickel electropolishing baths :which leave treated metal surfaces bright, highly reflective, and substantially free of oxide films and the like.

It is a still further object of the present invention to provide improved electropolishing baths for mckel capable of operating at comparatively high current densities, such as to achieve rapid leveling of surfaces being polished, and having broad operating ranges of anode current densities.

It is a still further object of the present invention to provide nickel-polishing baths having relatively broad ranges for the essential ingredients thereof, therebyassuring operativeness of the bath even though the composition of the bath may change somewhat during normal usage.

We have found that an electropolishing bath consisting essentially of hydrochloric acid in appreciable amounts and sulfuric acid is highly suitable for anodic polishing of nickel. The presence of relatively large percentages by volume of hydrochloric acid as an essential polishing ingredient in'an electropolishing bath is contrary to the teachings of the printed literature, which indicate that the presence of small amounts of hydrochloric acid is tion with sulfuric acid are advantageous in that they fa cilitate electropolishing of nickel parts at room temperature, require little or no temperature control, form no appreciable films or oxide coatings on polished parts, have a broad range of anode current densities, and are highly eificient in rapid leveling parts.

The above brief description as well as further objects, advantages and features of the present invention will be best appreciated by reference to the accompanied detailed description of presently preferred baths and treatment methods, when taken in conjunction with the accompanying drawings, wherein:

Figure 1 is a ternary diagram illustrating the electropolishing regions for the numerous solutions coming within the scope of the present invention; and

Figure 2 is a ternary diagram illustrating the lower operating limit of current densities for nickel electropolishing solutions according to the present invention.

Referring now specifically to Figure lot the drawings, there is illustrated a ternary diagram showing the relative proportions of hydrochloric acid, sulfuric acid and water for electropolishing solutions in accordance with the present invention. The three coordinates represent water which is present in increasing quantities along lines parallel to and progressively removed from the zero percent water axis 10; sulfuric acid which is present in increasing quantities along lines parallel to and progressively removed from the zero percent sulfuric acid axis 12; and hydrochloric acid which is present in increasing quantities along lines parallel to' and progressively removed from the zero percent hydrochloric acid axis 14. v

The solutions found suitable for electropolishing nickel are in the region approximately enclosed by the solid linesv in the triaxial diagram of Figure 1, specifically by the solid curved line AB, the solid straight line BC, thesolid curved line CD, the straight line DE, the solid curved line EF, andthe solid line'FA coextensive with the zero percent water axis."

Patented Apr. 1, 1958 still further object of the present invention to.

The approximate coordinates of the points defining the nickel ele'ctropolishing region are as follows: Azero percent water, 60 percent sulfuric. acid, and 40 percent hydrochloric acid.

B-32 percent water, 63 percent sulfuric acid, and percentfhydrochloricacid. i v C-70 percentwaterj25 percent sulfuric acid, and 5 percent hydrochloric'acid. D67.5 percent water, 5 percent sulfuric acid, and 27.5

percent hydrochloric acid. E59 percent water, 5 percent sulfuric acid, and'36 percent hydrochloric acid. i F-zerd percent water, '40 percent sulfuric acid, and 60 percent hydrochloric acid. Selection of a particular solution within the electropolishing regions depends upon practical considerations, such as the required (width of operating range and the power consumption. The wider the operating range, the more uniformwill be the polishing action on recessed or irregularly shaped objects, while the lower the total amount of current required, the smaller the power source. The specific baths listed below are illustrative of the wide variety of formulations which may be used in accordance with the present invention for the clectropolishing of nickel. It is to be expressly understood that no sharp transition occurs in the action of the electropolish'ing bath as the outer limits of the defined electropolishing region are approached. Rather the limits define a threshhold region wherein the polishing region may be other than optimum and/ or the power consumption prohibitive. Among the formulations recommended for electropolishing nickel are the following, which are listed in percentages by volume:

Of the above listed solutions, the solution No. 14 consisting essentially of 35 percent sulfuric acid, 35 percent hydrochloric acid and 30 percent water is found to be most suitable for the electropolishing of nickel. While the triaxial diagram shows the relative proportions of hydrochloric acid, sulfuric acid and water in a system consisting of only these three components, suitable'baths of these three components may include other ingredients such as other acids and/ or salts. However, the relative proportionsthat hydrochloric acid, sulfuric acid and water should bear to each other may be determined for operative and preferred ranges of compositions regardless of what other non-essential ingredients may be present in the baths, intentionally, due to contamination, or incidentally. For example, nickel, which has dissolved in the bath the to previous elcctropolishing, is an incidental ingredient.

Because of the'limited solubility of hydrochloric acid in sulfuric acid solutions, some of the illustrative baths are inherently unstable, although suitabl for elcctropolishing of nickel. Such inherent instubilny :seuts the problem of fuming and further causes slow changes in the composition of the electrolyte. Since the problem of fur-r ing presents an industrial hazard, it is desirable to employ solutions of a composition lying above the dash-dash line 16 on the ternary diagram of Figure 1 which dash-dash line is the theoretical solubility line for hydrogen chloride gas in sulfuric acid solutions. All solutions lying below thedash-dash line 16 are unstable although suitablefor electropolishing if lying within the prescribed regions; while solutions lying above the theoretical line 16 and within the solid lines defining the limits of electropolishing regions are stable and useful for polishing. The dashdash line 16 intersects the straight solid line BC at the point G and intersects the curved solid line EF at the point H.

Reference will now be made to Figure 2 which is a ternary diagram showing the lower operating range of current density in amperes per square foot for solutions lying within the electropolishing region illustrated in Figure 1. For solutions coming within the electropolishing region of Figure l the width of the operating range is in excess of 1,000 amperes per square foot, as measured from the lower operating limit determined by inspection of Figure 2. With respect to Figure 2, it is to be noted that the lower operating limit is broadly defined in each region and that the lower operating limit must be selected upon noting the relative position of the solution-to the several regions of different lower operating limits.

The conjoint reading of the diagrams illustrated in Figures 1 and 2 for a given solution in establishing the required operating current density should be well understood in the art. The following is for the purposes of illustration:

The point P on the ternary diagram of Figure l was previously indicated as solution No. 14 and has a composition of '30 percent water, 35'pcrcent sulfuric acid, and 35 percent'hydrochloric acid. In that the point P lies above the theoretical solubility line 16, this solution will be stable and not present the hazards of fuming. The correspondingpoint 'P lying on the diagram of lower operating limits illustrated in Figure 2 lies within the region having a lower operating limit of between 500 and 1,000 amperes. Thus the operating range for the solution P is in excess of 1,000 amperes per square foot (see Fig. 1), measured from a lower operating limit selected'between 500 and 1,000 amperes per square foot. In that the point P is substantially midway between the 500 and 1,000 gradients of the ternary diagram of Figure 2, the lower operating limit should be approximately 750 amperes with an operating range up to and including 1750 amperes per square foot.

Optimum results are obtained by operating at high current densities in that it is preferable to rapidly remove material in electropolishing. However, with the comparatively broad range of current densities, which are effective for electropolishing, the selection of the particular operating current is not critical and dilferent current densities may exist at various locations on an irregular object with the assurance thatthe object will be electropolished throughout the various portions of its surfaces.

Although the foregoing description is based upon compositions for producing highly lustrous polishes on nickel, equally within the contemplation of the disclosure is the polishing of electrodeposited nickel, non-ferrous nickel alloys and other allied materials having nickel as an essential ingredient.

Aconsideration of the ternary diagram of Fig. 1 will reveal that operative solutions in accordance with the present invention generally contain between 5 and 66% by volume of sulfuric acid, between 5 and 60% by volume of hydrochloric acid and less than 75% by volume of water. These numerical values are arrived at by mal:- ing straight lineapproximation of the various curved lines bounding the preferred area. For example, the maximum sulfuric acid content represented by the curved line AB corresponds,approximately to a straight line coinciding with the line indicating 66%-by volume of sulfuric acid; and the minimum sulfuric acid content represented by the line DE corresponds exactly to 5% by volume of sulfuric acid.

Numerous other modifications and substitutions in the present process may occur to those skilled in the art, and accordingly the appended claims should be given the latitude of interpretation consistent with the disclosure; at times certain features of the invention will be used without a corresponding use of other features.

What we claim is:

1. The method of electropolishing a nickel part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid which are present in relative percentages by volume lying within the area approximately enclosed in the ternary diagram of Figure 1 by the solid curved line AB, the solid straight line BC, the solid curved line CD, the solid straight line DE, the solid curved line EF, and the solid line FA which coincides with the zero percent water axis.

2. The method of electropolishing a nickel part including the steps of making said part the anode within an electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid which are present in relative percentages by volume lying within the area defined approximately in the ternary diagram of Figure 1 by the solid lines BC, CD, DE and EF, and the dash-dash line representing the theoretical solubility of hydrogen chloride, gas and sulfuric acid solution which dash-dash line intersects the solid line BC and the solid line EF, and passing electric current through said anode in an amount sufficient to obtain an electropolishing action of the surfaces of the nickel part.

3. The method of anodically polishing an article having nickel surfaces including the steps of making said article the anode in an electrolytic solution containing as essential ingredients from 5 to approximately 66 percent of sulfuric acid and from 5 to 60 percent of hydrochloric acid, said percentages being by volume in the relative percentages of said ingredients lying within the area defined in Figure 1 of the accompanying drawing by the solid line ABCDEFA and passing electric current through said anode in an amount sutlicient to obtain an electropolishing action of the surfaces.

4. The method of anodically polishing an article having nickel surfaces including the steps of making said article the anode in an electrolytic solution containing as an essential ingredient from 5 to approximately 66 percent sulfuric acid and from 5 to percent hydrochloric acid, said percentages being by volume and the relative percentages of said ingredients lying within the area defined in Figure 1 of the accompanying drawings by the lines AB, BC, DE, EF and FA, and passing an electric current through said anode in an amount sufiicient to obtain electric polishing action of said surfaces as determined by Figure 2 of the accompanying drawing which gives a value for the lower operating current density and within a range of 1,000 amperes per square foot from said lower current density.

5. The method of electropolishing a nickel part including the steps of making said part the anode in an aqueous electrolytic solution consisting essentially of sulfuric acid and hydrochloric acid, said solution containing between 5 and approximately 66% by volume of sulfuric acid, between 5 and 60% by volume of hydrochloric acid, and less than by volume of water, and passing electric current through said anode in an amount sufiicient to obtain an'e'lectropolishing action on the surfaces of said nickel part.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF ELECTROPOLISHING A NICKEL PART INCLUDING THE STEPS OF MAKING SAID PART THE ANODE IN AN AQUEOUS ELECTROLYTIC SOLUTION CONSISTING ESSENTIALLY OF SULFURIC ACID AND HYDROCHLORIC ACID WHICH ARE PRESENT IN RELATIVE PERCENTAGES BY VOLUME LYING WITHIN THE AREA APPROXIMATELY ENCLOSED IN THE TERNARY DIAGRAM OF FIGURE 1 BY THE SOLID CURVE LINE AB, THE SOLID STRAIGHT LINE BC, THE SOLID CURVE LINE CD, THE SOLID STRAIGHT LINE DE, THE SOLID CURVE LINE EF, AND THE SOLID LINE FA WHICH COINCIDES WITH THE ZERO PERCENT WATER AXIS. 